feat(build): build.zig MVP: build and run functionaltests on linux

NEW BUILD SYSTEM!

This is a MVP implementation which supports building the "nvim" binary,
including cross-compilation for some targets.
As an example, you can build a aarch64-macos binary from
an x86-64-linux-gnu host, or vice versa

Add CI target for build.zig currently for functionaltests on linux
x86_64 only

Follow up items:

-  praxis for version and dependency bumping
-  windows 💀
-  full integration of libintl and gettext (or a desicion not to)
-  update help and API metadata files
-  installation into a $PREFIX
-  more tests and linters

test/functional/api/vim_spec.lua

@@ -4854,7 +4854,7 @@ describe('API', function()
       -- #20681
       eq('Invalid command: "win_getid"', pcall_err(api.nvim_cmd, { cmd = 'win_getid' }, {}))
       eq('Invalid command: "echo "hi""', pcall_err(api.nvim_cmd, { cmd = 'echo "hi"' }, {}))
-      eq('Invalid command: "win_getid"', pcall_err(exec_lua, [[return vim.cmd.win_getid{}]]))
+      matches('Invalid command: "win_getid"$', pcall_err(exec_lua, [[return vim.cmd.win_getid{}]]))
 
       -- Lua call allows empty {} for dict item.
       eq('', exec_lua([[return vim.cmd{ cmd = "set", args = {}, magic = {} }]]))
@@ -4862,16 +4862,16 @@ describe('API', function()
       eq('', api.nvim_cmd({ cmd = 'set', args = {}, magic = {} }, {}))
 
       -- Lua call does not allow non-empty list-like {} for dict item.
-      eq(
-        "Invalid 'magic': Expected Dict-like Lua table",
+      matches(
+        "Invalid 'magic': Expected Dict%-like Lua table$",
         pcall_err(exec_lua, [[return vim.cmd{ cmd = "set", args = {}, magic = { 'a' } }]])
       )
-      eq(
-        "Invalid key: 'bogus'",
+      matches(
+        "Invalid key: 'bogus'$",
         pcall_err(exec_lua, [[return vim.cmd{ cmd = "set", args = {}, magic = { bogus = true } }]])
       )
-      eq(
-        "Invalid key: 'bogus'",
+      matches(
+        "Invalid key: 'bogus'$",
         pcall_err(exec_lua, [[return vim.cmd{ cmd = "set", args = {}, mods = { bogus = true } }]])
       )
     end)